1. Field of the Invention
The present invention relates to an image forming apparatus which primarily transfers a toner image formed on a photoreceptor onto an intermediate transfer member and then secondarily transfers the toner image on the intermediate transfer member onto a recording sheet.
2. Description of the Related Art
In recent years, electrophotographic image-forming apparatuses have been widely used in which a toner image formed on a photoreceptor is primarily transferred onto an intermediate transfer member, and the toner image on the intermediate transfer member is then secondarily transferred onto a sheet. When the distance between a secondary transfer portion and a fixing portion in such an image-forming apparatus is short, a recording sheet may extend beyond the secondary transfer portion and the fixing portion. As such, the rotating speed of the intermediate transfer member and the rotating speed (i.e., the fixing speed) of the fixing portion must correlate during the secondary transfer.
An image formed on a recording sheet such as a sheet of thick paper, an envelope, a postcard, an OHP sheet, a label sticker, a tabbed sheet, or a tracing paper cannot be satisfactorily fixed at a fixing speed applied to a sheet of normal paper. Instead, the fixing speed applied to the predetermined sheets mentioned above must be reduced relative to the fixing speed applied to a sheet of normal paper. Accordingly, in the case of forming an image on a recording sheet such as a sheet of thick paper, an envelope, a postcard, an OHP sheet, a label sticker, a tabbed sheet, or a tracing paper, the rotating speed of the intermediate transfer member must also be reduced so as to correlate with the reduced fixing speed, thereby leading to a decrease in productivity.
Japanese Patent Laid-Open No. 4-67174 discloses an image-forming apparatus in which the intermediate transfer member is rotated at a predetermined rotating-speed during primary transfer. Immediately after completion of the primary transfer, the rotating speed of the intermediate transfer member is reduced to the fixing speed so as to perform the secondary transfer. With this configuration, a decrease in productivity during the primary transfer can be minimized.
In the above-mentioned image-forming apparatus, however, it takes a long period of time after the speed reduction of the intermediate transfer member to start of the secondary transfer, thereby decreasing productivity.
Also, Japanese Patent Laid-Open No. 9-146434 discloses an image-forming apparatus in which the distance between primary and secondary transfer portions of the intermediate transfer member is set longer than the maximum length of any image. The speed reduction of the intermediate transfer member starts when the front edge of an image formed on the intermediate transfer member reaches a predetermined distance from a secondary transfer position and finishes before the front edge reaches the secondary transfer position.
The above image-forming apparatus works satisfactorily on the premise that the distance between the primary and secondary transfer portions of the intermediate transfer member is equal to or greater than the sum of the maximum length of any one image and a length needed for the speed reduction of the intermediate transfer member. Hence, Japanese Patent Laid-Open No. 9-146434 proposes that the distance between the primary and secondary transfer portions of the intermediate transfer member be configured longer in order to form a longer image.
Unfortunately, in some image-forming apparatuses, the distance between the primary and secondary transfer portions of the intermediate transfer member cannot be configured longer because of structural restrictions. Also, in some image-forming apparatuses in recent years, a plurality of images is formed on the entire loop of the intermediate transfer member. In such an image-forming apparatus, complete speed reduction of the intermediate transfer member depends on the size of an image and the number of images formed on the entire loop of the intermediate transfer member. Accordingly, if the secondary transfer is always performed when the front edge of an image formed on the intermediate transfer member reaches the secondary transfer portion after finish of the primary transfer, the secondary transfer may start before the speed reduction of the intermediate transfer member is completed and is therefore performed in a state in which the rotating speed of the intermediate transfer member is higher than the fixing speed.